1. Field of the Invention
The present invention relates to a brazing sheet of aluminum alloy to be used for heat exchangers and the like.
2. Description of the Related Art
A heat exchanger mounted on an automobile is produced by forming a brazing sheet of aluminum alloy into a prescribed shape and then assembling and brazing the formed brazing sheet. Brazing sheets for tubes recently show a trend toward thickness reduction from 0.3-0.5 mm to 0.2 mm or less for lighter heat exchangers. Thinner brazing sheets require higher strength and better corrosion resistance.
One way to reduce the weight of heat exchangers is by using bare fins (not cladded with brazing filler metal). Bare fins are attached to tubes whose surface is cladded with brazing filler metal, and this poses a problem with insufficient corrosion resistance.
To address this problem, there has been proposed a brazing sheet which exhibits good corrosion resistance after brazing on the surface cladded with brazing filler metal. This brazing sheet is composed of a core material of Al—Mn—Cu alloy and a layer of brazing filler metal formed thereon from an Al—Si alloy containing Zn, the latter providing sacrificial protection to the brazed surface. (See Japanese Patent No. 3360026.) According to this patent, corrosion resistance is improved by diffusion of Zn from the brazing filler metal to the core material at the time of brazing, which makes the potential of the brazed surface less noble, thereby providing sacrificial protection.
Unfortunately, the technology disclosed in the above-mentioned patent has the disadvantage that diffusion of Zn leaves only a small amount of Zn in the brazed surface which is not enough to produce a sufficient potential between the brazed surface and the center (in the thickness direction) of the brazing sheet.
Another disadvantage is that Cu diffuses from the core material to the brazing filler metal at the time of brazing and the thus diffused Cu forms a concentrated layer on the brazed surface in the cooling stage that follows brazing. This makes the potential of the brazed surface noble and hence prevents a sufficient potential difference from occurring between the brazed surface and the center of the brazing sheet. As the result, the brazing sheet exposed to a severe corrosive environment suffers pitting soon on account of insufficient sacrificial protection.
In addition, the brazing filler metal that flows at the time of brazing contains a large amount Zn and hence the fillet (brazed part) readily suffers preferential corrosion. This raises the possibility of brazed bare fins peeling off early.
The present invention was completed in view of the foregoing. Thus, it is an object of the present invention to provide a brazing sheet of aluminum alloy which exhibits good corrosion resistance (on the surface cladded with brazing filler metal) after brazing and also exhibits good brazeability.